122 



TEE ANATOMY OF VERTEBRATED ANIMALS. 



which not only overlap, but are fitted together by pegs and 

 sockets, where their anterior and posterior edges come iuio 

 contact. 



Fig. 3S. — The brain of Lepidoatens seTniradJatus. A. From above; 5. From below./ 

 the medulla oblongata ; d, the cei-eboUum ; c, the optic lobes of the mesenceiiha!<in . {/^ 

 the cerebral hemispheres ; h, the pituitary b^dy ; i, the lobi inferiores. t7/i, the chlad- 

 ma ; I., olfactory ; II., optic nerves. 



The endoskeleton is not less diversely modified; and it la 

 worthy of remark that no sort of relation, either direct or 

 inverse, is traceable between the completeness of the endo- 

 skeleton and that of the exoskeleton. Thus Spatularia, 

 Scapirhynchus, and Accipenser have a persistent notochord, 

 in the sheatb of which mere cartilaginous rudiments of the 

 arches of vertebrae appear. The ribs, when present, are par- 

 tially ossified. Polypterus and Amia have fully ossified ver- 

 tebrae, the centra of which are amphicoelous. Lepidosteus alsc 

 has fully-ossified vertebrie ; but their centra are opisthocce- 

 lous, having a convexity in front and a concavity behind, as in 

 some Amphibia. 



More or fewer of the anterior vertebrae, or their cartilagi- 

 nous representatives, are united witb one another, and with 

 the posterior part of the skull. And the cranium may consist 

 principally of cartilage, membrane bones being superadded ; 

 or the primordial cartilage maybe largely superseded bj' bone, 

 as in the Teleostei. 



Spatularia^ Scapiirhynchus, and Accipenser, have skulls of 

 the former description. The cranium is one mass of cartilage, 

 continuous behind with the coalesced anterior spinal cartilages, 

 so as to be immovably connected with the spinal column. The 

 notochord enters its base, and terminates in a point behind 

 the pituitary fossa. In front, the cartilage is produced into a 



